Rdio may be on the path to a free, ad-supported streaming music service thanks to a deal with radio behemoth Cumulus.

According to the New York TImes, Cumulus will sell the advertising for a free version of Rdio in the United States, and will promote the service across its 525 radio stations. In exchange, Cumulus will get a significant equity stake in Rdio’s parent company, Pulser Media. Cumulus is the second-largest radio operator in the United States behind Clear Channel.

Rdio’s current service costs $5 per month for desktop access, and $10 per month with phone and tablet access. Similar to Spotify, Rdio offers millions of on-demand songs with no listening limits. You can create your own playlists and collections, listen to entire albums or create “stations” tailored to your interests.

The Times‘ report doesn’t say what Rdio’s free version will look like. Rdio could offer free, on-demand streaming on desktop PCs, but I wouldn’t rule out the possibility of a more limited radio-like service, similar to Pandora. Perhaps Rdio will adopt Spotify’s approach of offering free on-demand listening on desktops, and just the radio-like stations on phones and tablets. All we know for sure is that Cumulus will offer its own playlists on Rdio, based on its existing stations and syndicated shows. (I’ve reached out to Rdio for more details but haven’t heard back.)

Update: An Rdio representative tells me the company’s engineers are still working on what the experience will look like. Rdio is aiming to launch a free version of its service by the end of the year, but it could take longer.

Although I’ve tried to sing the praises of Rdio to whoever will listen, it’s a tough sell when there’s no free version to lure people in. Rdio does offer an extended free trial on desktop web browsers, providing “up to” six months of music depending on how much you use it. But that’s still no match for Spotify, whose free desktop app and website don’t come with an expiration date.

Still, Rdio’s apps are well-designed with useful features, like the ability to use your phone as a remote control for playback on another device. And I like how Rdio lets you create a “collection” of favorite artists, albums and songs, instead of the basic playlists and starred tracks that Spotify offers. If you’re looking to replace your record collection with streaming music, Rdio is a solid option.

We’ll have to wait for more details, but I’m cautiously optimistic that deal will help Rdio get the attention it deserves. Not all streaming music acquisitions turn out well for users (see: Mog, Napster), but hopefully Cumulus can bring in more users and help Rdio thrive.